A lighting beam with cut-off is to be understood to mean a lighting beam which has a directional limit or cut-off line above which the intensity of the light emitted is weak. The functions of passing or dipped-beam lights, and anti-fog lights, are examples of light beams with cut-off in accordance with current European legislation.
Generally, in an elliptical headlight, the cut-off is achieved by means of a mask, which is formed from a vertical plate the profile of which is suitably adapted, and which is interposed axially between the elliptical reflector and the convergent lens, the mask being arranged in the vicinity of the second focus of the reflector.
The mask occults the light rays issued from the light source, which are reflected by the reflector towards the lower part of the focal plane of the convergent lens and which would, in the absence of the mask, be emitted by the headlight above the cut-off line.
However, such a solution does have certain difficulties.
Thus, one disadvantage of this type of headlight is that a significant part of the light flux emitted by the light source is dissipated in the rear face of the mask.
Another solution consists in making a lighting module which makes use of a light source and a Fresnel optic, or a reflector of the complex surface type. In order to create a cut-off it is necessary to align the edges of the images of the light source on the measuring screen which is used for statutory testing of the lighting beam.
This solution again has certain problems.
Thus, where the light source is a diode, it is very difficult to produce a clean cut-off. In this connection, the image of the virtual source that corresponds to the diode is generally round and is diffuse, and it is far more complicated to produce a clean cut-off by aligning the corresponding images of round form.
This difficulty can be overcome by making use of a diaphragm with the diode, but a large quantity of the light energy produced by the diode is then lost.
In addition, those emission indicators of the diodes that are known to have the best performance are complex, and the production of a homogeneous beam is very difficult to obtain from direct images of the diode.